Add docs for MVVMTK0039

docs/mvvm/generators/errors/MVVMTK0039.md

@@ -0,0 +1,54 @@
+---
+title: MVVM Toolkit warning MVVMTK0039
+author: Sergio0694
+description: MVVM Toolkit warning MVVMTK0036
+keywords: community toolkit, dotnet, csharp, mvvm, net core, net standard, source generators
+dev_langs:
+  - csharp
+---
+
+# MVVM Toolkit warning MVVMTK0039
+
+Asynchronous methods annotated with `[RelayCommand]` should return a `Task` value, and not be `async void`. This allows the generator to generate asynchronous commands instead (ie. `AsyncRelayCommand` or `AsyncRelayCommand<T>`), which offer additional features such as concurrency control, tracking execution, and more.
+
+The following sample generates MVVMTK0039:
+
+```csharp
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+
+namespace MyApp;
+
+public partial class SampleViewModel : ObservableObject
+{
+    [RelayCommand]
+    private async void DoWorkAsync()
+    {
+        await Task.Delay(1000); // Do some work
+    }
+}
+```
+
+You can see that `DoWorkAsync` is asynchronous, but is just returning `void`. You can fix this by updating the code as follows:
+
+```csharp
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+
+namespace MyApp;
+
+public partial class SampleViewModel : ObservableObject
+{
+    [RelayCommand]
+    private async Task DoWorkAsync()
+    {
+        await Task.Delay(1000);
+    }
+}
+```
+
+## Additional resources
+
+- You can find more examples in the [unit tests](https://github.com/CommunityToolkit/dotnet/tree/main/tests/CommunityToolkit.Mvvm.SourceGenerators.UnitTests).

docs/mvvm/generators/errors/TOC.yml

@@ -71,6 +71,8 @@ items:
   href: MVVMTK0035.md
 - name: MVVMTK0036
   href: MVVMTK0036.md
+- name: MVVMTK0039
+  href: MVVMTK0039.md
 - name: MVVMTKCFG0001
   href: MVVMTKCFG0001.md
 - name: MVVMTKCFG0002